MAX783CSLASHD datasheet(12/28 Pages) MAXIM

12 / 28 page

Triple-Output Power-Supply Controller

for Notebook Computers

______________________________________________________________________________________

Soft-Start/SS_ Inputs

Connecting capacitors to SS3 and SS5 allows gradual

build-up of the +3.3V and +5V supplies after ON3 and

ON5 are driven high. When ON3 or ON5 is low, the

appropriate SS capacitors are discharged to GND.

When ON3 or ON5 is driven high, a 4µA constant cur-

rent source charges these capacitors up to 4V. The

resulting ramp voltage on the SS_ pins linearly increas-

es the current-limit comparator setpoint so as to

increase the duty cycle to the external power MOSFETs

up to the maximum output. With no SS capacitors, the

circuit will reach maximum current limit within 10µs.

Soft-start greatly reduces initial in-rush current peaks

and allows start-up time to be programmed externally.

Synchronous Rectifiers

Synchronous rectification allows for high efficiency by

reducing the losses associated with the Schottky recti-

fiers. Also, the synchronous rectifier MOSFETS are

necessary for correct operation of the MAX783's boost

gate-drive and VDD supplies.

When the external high-side power MOSFET turns off,

energy stored in the inductor causes its terminal volt-

age to reverse instantly. Current flows in the loop

formed by the inductor, Schottky diode, and load—an

action that charges up the filter capacitor. The Schottky

diode has a forward voltage of about 0.5V which,

although small, represents a significant power loss and

degrades efficiency. A synchronous rectifier MOSFET

parallels the diode and is turned on by DL3 (or DL5)

shortly after the diode conducts. Since the on resis-

tance (rDS(ON)) of the synchronous rectifier is very low,

the losses are reduced.

The synchronous rectifier MOSFET is turned off when

the inductor current falls to zero.

Cross conduction (or “shoot-through”) occurs if the high-

side switch turns on at the same time as the synchronous

rectifier. Internal break-before-make timing ensures that

shoot-through does not occur. The Schottky rectifier con-

ducts during the time that neither MOSFET is on, which

improves efficiency by preventing the synchronous-rectifi-

er MOSFET’s lossy body diode from conducting.

The synchronous rectifier works under all operating condi-

tions, including discontinuous-conduction and idle-mode.

The +3.3V synchronous rectifier also controls the 15V VDD

voltage (see the

High-Side Supply (VDD) section).

Boost Gate-Driver Supply

Gate-drive voltage for the high-side N-channel switch is

generated with a flying-capacitor boost circuit as shown

in Figure 4. The capacitor is alternately charged from

the VL supply via the diode and placed in parallel with

the high-side MOSFET’s gate-source terminals. On start-

up, the synchronous rectifier (low-side) MOSFET forces

LX_ to 0V and charges the BST_ capacitor to 5V. On the

second half-cycle, the PWM turns on the high-side

MOSFET by connecting the capacitor to the MOSFET

gate by closing an internal switch between BST_ and

DH_. This provides the necessary enhancement voltage

to turn on the high-side switch, an action that “boosts”

the 5V gate-drive signal above the battery voltage.

Ringing seen at the high-side MOSFET gates (DH3 and

DH5) in discontinuous-conduction mode (light loads) is

a natural operating condition caused by the residual

energy in the tank circuit formed by the inductor and

stray capacitance at the LX_ nodes. The gate driver

negative rail is referred to LX_, so any ringing there is

directly coupled to the gate-drive supply.

Modes of Operation

PWM Mode

Under heavy loads—over approximately 25% of full

load—the +3.3V and +5V supplies operate as continu-

ous-current PWM supplies (see

Typical Operating

Characteristics). The duty cycle (%ON) is approximately:

%ON = VOUT/VIN

Current flows continuously in the inductor: First, it ramps

up when the power MOSFET conducts; then, it ramps

down during the flyback portion of each cycle as energy

is put into the inductor and then discharged into the load.

Note that the current flowing into the inductor when it is

being charged is also flowing into the load, so the load is

LEVEL

TRANSLATOR

PWM

BST_

DH_

LX_

DL_

BATTERY

INPUT

Figure 4. Boost Supply for Gate Drivers

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